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Monday, June 13, 2011

Using the BGP ASN from multiple locations

Can the same AS number be used by a organization at two different sites to peer with two different ISPs?The scenario is as per what is shown below: ASN 1111-a (100.100.100.0/24)---ASN XXX---ASN YYY---ASN 1111-b (200.200.200.0/24)The AS number is 1111. The organization using it at location A to advertise network 100.100.100.0/24 to the ISP peer. At location B the organization is is using the same ASN to advertise 200.200.200.0/24 to another ISP. There is no peering between the two AS 1111 routers as they are physically separated.

It is very common for a single organization to have multiple routers running BGP to route with several different iSPs. In this case the organization will use the same AS number on each of its routers running BGP.Note that when the organization does this there should be a BGP session between the organization's own routers (running IBGP between its routers). It does not matter that they are physically separated - the organization's routers should run BGP between its own routers.

You need to advertise separate public ip address blocks out of the two sites.If you try to advertise something more specific that a /24 you may have problems with summarization of your routes at the ISP level.So it can work in real world if you have at least two /24 public ip address blocks registered with your AS numberWithout knowing the details of the remote site routes it is difficult to say what is happening when something goes wrong.The BGP rule that says an AS will not accept routes that have it's own ASN in the routeThis can be overriden with the command neigh x.x.x.x allowas-in #timeswhere x.x.x.x is the ISP neighborYou can still run an iBGP session between the two sites using the public ip addresses over the ISP links just to get the details of routes on the other site.

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